Garner, the star of ABC's "Alias," was in Canada for the filming of "Catch and Release" while Affleck was wrapping shooting on "Truth, Justice and the American Way."
"She's starting to show -- they have to get married soon!," In Touch magazine's Tom O'Neil said.
"We predict once production wraps they're going to get married quietly."
Affleck proposed to Garner at an April bash in Brentwood, Calif., to celebrate Garner's 33rd birthday.
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